The present invention relates to the field of gas turbine engines and, in particular, to devices and methods directed towards cooling the walls, shells, and liners of combustion chambers used in gas turbine engines.
Gas turbine engines must operate at high operating temperatures to maintain an acceptable efficiency. As the operating temperature of the engine is increased, the efficiency of the engine improves. However, as operating temperatures increase, the operational lifetime of the engine components decreases because of increased thermal stress. A component of particular interest is the combustion chamber, or combustor, of the engine. One way to increase the durability of the combustor is to use an extremely thick shell or liner, but this increases weight, which is undesirable, particularly in aircraft. A more common way is to use some form of wall cooling. It is well known in the art to use cooling tubes along the surface of the combustor walls in order to reduce the wall temperature and to increase the lifetime of the chamber. The cooling tube directs cooling air along the surface of the combustor wall to cool the wall either by conduction of excess heat away from the wall, by keeping hot combustion gasses from directly contacting the wall, or by both. Such devices may also direct the cooling air counter to the direction of flow of combustion gases out of the chamber.
The use of cooling tubes for cooling combustors is well known to the art. For example, U.S. Pat. No. 4,288,980 discloses a plurality of circumferentially spaced cooling tubes for use on the interior of the combustor. The cooling tubes extend from one end of the combustor to the other in a linear manner. U.S. Pat. No. 4,607,487 discloses a combustor in which the combustion wall has a plurality of generally cylindrical passages built into the combustor wall itself, where the passages feature a way of causing vortices therein to disrupt laminar flow of the cooling air through the passages. These passages also are described as being linear. U.S. Pat. No. 5,724,816 discloses a cooling structure for a combustor with a plurality cooling channels extending axially and in some cases circumferentially to create cross flow passages between the axial channels. Again, the channels are shown as being linear.
As an example, a prior art cooling tube 210, as shown in FIG. 1, is configured on a curved surface 220 that is curved in all three dimensions. Regardless of the orientation of the prior art cooling tube 210 on the curved surface 220, the centerline 230 of the prior art cooling tube 210 can always be projected onto a plane 240 such that the centerline 230 and the endpoints 211, 212 of the prior art cooling tube 210 all lie in a straight line in the plane 240. When the curved surface 220 is a liner of a combustor, one end 211 of the prior art cooling tube 210 terminates at one end of the combustor and the opposing end 212 of the prior art cooling tube 210 terminates at the other end of the combustor. A cooling gas, normally pressurized air, flowing through the prior art cooling tube 210 absorbs heat from the walls of the combustor. The prior art cooling tube 210 also absorbs heat. Depending upon the coefficient of expansion of the material from which the prior art cooling tube 210 is made, it may expand or contract in response to the steep temperature gradient, resulting in stress placed upon the cooling tube itself or at its attachment points.
The contact of the cooling tube with the cooling air running through the tube and with the hot combustion gasses surrounding the tube results in severe thermal stress to the cooling tube. This, in turn, produces elastic expansion and contraction of the cooling tube as it is subjected to high temperature gradients. If the tube is rigidly attached to the combustor wall, it may deform or crack from such stresses, resulting in a decreased time between maintenance actions. One solution known to the art is to construct the cooling tubes and chamber walls from ceramic materials that have a low coefficient of expansion. But even with the lower thermal expansion these materials exhibit brittleness and lower thermal conductivity and are therefore still susceptible to stresses generated by the thermal expansion differences between the tubes and the chamber.
It can thus be seen that there is a need for a cooling tube for a combustor in a gas turbine engine that has improved properties of resistance to thermal expansion and contraction and to vibration.